As disclosed by this inventor of the present invention in the U.S. Pat. No. 5,085,660, a vertebral fixation system comprises a double-threaded bone screw having a first threaded portion fro fastening onto a bone or vertebra, and a second threaded portion engageable with a fixation plate. The second threaded portion and the fixation plate are fastened at a right angle at which the first threaded portion is fastened onto a bone or vertebra. As a result, the angle at which the first threaded portion is fastened onto a bone or vertebra can not be changed in accordance with the surgical requirement when the fixation plate is chosen. Furthermore, the fixation plate is more rigid than the bone or vertebra onto which the double-threaded bone screw is fastened, thereby forcing the first threaded portion to fasten onto the bone or vertebra according to the advancing angle at which the second threaded portion is engaged with the fixation plate, if the angle at which the first threaded portion is fastened onto a bone or vertebra deviates from the advancing angle of the second threaded portion in the fixation plate. As a result, the threads formed by the first threaded portion in the bone or vertebra are vulnerable to damage and the first threaded portion of the double-threaded bone screw is therefore unable to hold the bone or vertebra securely.